There are several reasons that ink-jet printing has become a popular way of recording images on various media surfaces, particularly plain paper. Some of these reasons include low printer noise, capability of high-speed recording, and multi-color recording. Additionally, these advantages can be obtained at a relatively low price to consumers. However, though there has been great improvement in ink-jet printing, accompanying this improvement are increased demands by consumers in this area, e.g., higher speeds, higher resolution, full color image formation, increased stability, higher image quality, etc.
As new ink-jet inks are developed, there are several traditional characteristics that are considered when evaluating the ink for use in conjunction with a printing surface or other inks. Such characteristics include edge acuity and optical density of the image on the surface, black to color bleed control, black to color wicking, halo control, dry time of the ink on the substrate, adhesion to the substrate, lack of deviation in ink droplet placement, presence of all dots, resistance of the ink after drying to water and other solvents, long term storage stability, and long term reliability without corrosion or nozzle clogging. Though the above list of characteristics provides a worthy goal to achieve, there are difficulties associated with satisfying all of the above characteristics. Often, the inclusion of an ink component meant to satisfy one of the above characteristics can prevent another characteristic from being met. Thus, most commercial inks for use in ink-jet printers represent a compromise in an attempt to achieve at least an adequate response in meeting all of the above listed requirements.
One characteristic of many ink-jet inks is the tendency of the ink to mix with or run into an adjacently printed color. This characteristic is undesirable and is known as “bleed.” The degree of bleed is apparent when printed black ink bleeds into an adjacent colored ink, and is typically manifested in the appearance of the black ink spreading into the colored ink. There are other related print phenomena that reduce print quality which are also occur when printing black and colored inks adjacently, such as halo and wicking. Halo effect relates to a white or gray band that can appear at the interface of black and colored inks which occurs as a result of liquid or colorant migration. Similarly, wicking refers to migration of ink from an image along paper fibers, thereby creating a ragged edge. Both of these phenomena, along with bleed, diminish print quality generally.
A variety of methods have been explored to achieve bleed control, reduction in halo effect, and/or reduction in wicking with varying degrees of success. For example, both reactive and non-reactive bleed control systems have been used to decreased bleed to some degree. However, most of these methods have focused on enhancing precipitation of the inks upon printing. Although effective with respect to bleed, this property also reduces the long term stability and reliability of the nozzle as it increases clogging and aggregation before printing. Typically, the addition of certain dispersants within the black ink can improve long term stability, but can also decrease bleed control. Other methods used to control bleed have included using salts, pH differentials, or cosolvents which are immediately removed upon printing. Methods which utilize salt mechanisms and/or pH differentials generally require that the colored inks used in conjunction with the black inks have certain chemical components or properties such as a salt or low pH. These types of methods are referred to as reactive bleed control. Accordingly, investigations continue into developing ink formulations and ink combinations that have improved properties and that do not improve one property at the significant expense of the others.